<!DOCTYPE html>
<html>

<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>draggable</title>
  <script src="./draggable.bundle.js"></script>
  <style>
    .item {
      height: 100%;
    }

    .dropzone {
      outline: solid 1px;
      height: 50px;
    }

    .draggable-dropzone--occupied {
      background: lightgreen;
    }
  </style>
</head>

<body>
  <div class="container">
    <div class="dropzone draggable-dropzone--occupied">
      <div class="item">A</div>
    </div>
    <div class="dropzone draggable-dropzone--occupied">
      <div class="item">B</div>
    </div>
    <div class="dropzone draggable-dropzone--occupied">
      <div class="item">C</div>
    </div>
  </div>

  <div class="container">
    <div class="dropzone"></div>
  </div>

  <script>
    const droppable = new Draggable.Droppable(document.querySelectorAll('.container'), {
      draggable: '.item',
      dropzone: '.dropzone'
    });

    droppable.on('droppable:dropped', () => console.log('droppable:dropped'));
    droppable.on('droppable:returned', () => console.log('droppable:returned'));
  </script>

</body>

</html>